The invention relates to a method, of converting a sequence of m-bit information words to a modulated binary signal, where m is an integer, in which method an n-bit code word is delivered for each received information word from the sequence, where n is an integer exceeding m, and the delivered code words are converted to the modulated signal. The modulated signal comprises bit cells having a first signal value and bit cells having a second signal value, and comprises for each of the delivered code words a corresponding signal portion. In accordance with the method, when one of the code words is assigned to one of the information words to be converted, that code word is selected from a set of code words, which set depends on a coding state determined after a code word has been delivered and which state is related to a digital sum value at the end of the modulated signal portion that corresponds to the delivered code word. The digital sum value denotes for a directly preceding portion of the modulated signal a running value of a difference between the number of bit cells having the first signal value and the number of bit cells having the second signal value.
The invention further relates to a method of producing a record carrier on which a modulated signal obtained according to the previous method is recorded.
The invention also relates to a coding device for converting a sequence of m-bit information words to a modulated binary signal, where m is an integer. Such a device comprises an m-to-n converter for converting each of the m-bit information words from the sequence to an n-bit code word, where n is an integer exceeding m, another converter for converting the code words converted by the m-to-n converter to the modulated signal, which signal comprises bit cells having a first signal value and bit cells having a second signal value, and comprises for each of the delivered code words a corresponding signal portion, and a determining unit for determining a coding state related to a digital sum value at the end of the modulated signal portion that corresponds to the delivered code word, which digital sum value denotes for a directly preceding modulated signal portion a running value of a difference between the number of bit cells having the first signal value and the number of bit cells having the second signal second value, wherein the m-to-n converter comprises a selection for selecting for the conversion a code word from a set of code words that depends on the coding state.
The invention further relates to a recording device in which such a coding device is used.
The invention still further relates to a signal comprising a sequence of q successive information signal portions which represent q information words, where q is an integer, in which signal each of the information signal portions comprises n bit cells which have a first or a second signal value, where each information signal portion belonging to a predefined group of information signal portions uniquely establishes an information word, and the number of successive bit cells having the same signal value is greater than or equal to 1 and smaller than or equal to k, where k is an integer. Each in information signal portion belonging to a second group of information signal portions with an adjacent information signal portion uniquely establishes an information word.
The invention further relates to a record carrier on which that signal is recorded.
The invention furthermore relates to a decoding device for converting that signal to a sequence of m-bit information words. The device comprises converter for converting the signal to a bit string of bits having a first or a second logical value, which bit string contains n-bit code words corresponding to the information signal portions, and another converter for converting the sequence of code words to the sequence of information words, where each of the code words to be converted is assigned an information word that depends on the code word to be convened.
Finally, the invention relates to a reading device in which a record carrier of this type is used.
Such methods, such devices, such a record carrier and such a signal are known from EP-A 0.150.082.
That document describes a modulation system in which a sequence of 8-bit information words is converted to a sequence of 10-bit code words. The 10-bit code words are converted to a modulated signal formed by bit cells having a first or a second signal value. Each bit cell represents a bit from the 10-bit code word sequence, where the logical value of the bit is denoted by the signal value of the bit cell. On conversion, each time a 10-bit code word is delivered, a digital sum value is computed for the code words already delivered. This digital sum value denotes the difference between the number of "0" bits and the number of "1" bits for the delivered part of the code word sequence.
The code word to be delivered is selected from a set of code words that depends on the computed digital sum value. The code words in the set are selected in such a way that the digital sum value remains within a small range, which leads to the fact that the frequency spectrum of the signal does not contain frequency components in the low-frequency area. Such a signal is also referenced a DC-free signal or DC-balanced signal. The lack of low-frequency components in the signal generally has great advantages for information transfer via a record carrier or other transmission channel.
In information recording there is always a need for increasing the information density on the record carrier.
A possible solution is a reduction of the number of bit cells in the modulated signal for each information word.
The problem with this is that as a result of the reduction of the number of bit cells for each information word, the number of unique bit combinations by which the information words can be represented diminishes, meaning in that less stringent restrictions can be made on the modulated signal, for example, restrictions relating to low-frequency contents of the modulated signal.